It is well known that three basic types of electrically driven vibrators generally exist. They are the environmental laboratory shaker, the concentric weight driven by a motor, and the electromagnet.
The environmental laboratory type shaker is analogous to a very large high fidelity speaker. The level and frequency of the vibration is determined by a control unit that drives the shaker through a powerful amplifier. Small test samples may be mounted directly to the shaker head. Larger samples are bolted to a slip-plate which in turn is usually supported by an oil bearing mounted on a concrete block. The shaker head is attached to the slip-plate which then drives the slip-plate. This system is capable of a wide range of levels and frequencies. It is expensive to acquire and to operate, and it is bulky. Once installed, it is moved only with great difficulty and expense.
The concentric weight vibrator comprises an electric motor with an offset weight or weights mounted to its output shaft. The rotating unbalanced shaft causes the entire motor to vibrate. The device may be mounted to anything having sufficient strength to sustain the weight and vibration and having sufficient room to accommodate its bulk. The frequency of vibration is dependent on the speed of the motor, while the level of vibration is determined by the amount of offset of the weight and the amount of weight, as well as motor speed. This unit has many applications. Larger units are used in industry on polishing, deburring and grinding equipment, and on parts conveyers, while smaller units are used in home massage equipment such as vibrating beds and chairs, as well as foot massagers. In the latter case, any unit using this type of vibrator must be thick enough to accommodate its bulk. Even miniature motors require space that prevents the development of a low profile platform preferably of less than one-half to one inch thick.
The electromagnet type vibrator comprises a coil of wire wound on a core of permeable metal and an armature. When current is applied to the coil a magnetic field is generated. The coil is located in close proximity to a ferrous metal armature that is attracted toward the magnetic field. When the electric current stops, the magnetic field collapses and the metal armature returns to its original position. The level of the vibration is dependent on the amount of current through the coil while the frequency is determined by the on-off cycle of the current. In most applications current is supplied by a 115V a.c. line. A diode is connected in series with the coil, thus providing a pulsating current at 60 hertz. This vibrator is used in many of the same applications as the concentric weight vibrator, and with the same limitations.